Added support for HH:MM on session, study and break timers.
Cleaned up timer input code to avoid code repetition.

Will support more timers being added in the future, with little to change (other than adding buttons to array and adding an action).
Added validation to input fields.

Prevents force closing when nothing is entered in fields. Also ensures that timers with 0 seconds aren't added - or we'd be stuck in an indefinite loop between timers.
Cleaned up the UI.

Added hours and minutes on the same row. Grouped each timer into logical blocks. Updated text on screen; now much more user friendly.
Fixed incorrect container IDs. Sorry!
Merged prototype into master, as that branch is finished with.